Use FreeType macros for tttables.h inclusion

As of FreeType 2.1.6 (November 2003), using #include to include Freetype libraries directly is not supported.

This has come to a head, as in FreeType 2.5.0, the location of headers has been moved around, breaking building of libgdiplus.

This slight change uses the "official" way to include the required header file, without breaking building on older versions of the library.
Merge pull request #14 from directhex/master

Use FreeType macros for tttables.h inclusion